Set Ubuntu version to 20.04 in CI.

.github/workflows/ci.yml

@@ -18,7 +18,7 @@ jobs:
   # versions of Python.
   #
   unit-tests:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       max-parallel: 5
       matrix:
@@ -51,7 +51,7 @@ jobs:
           tox -e py
 
   mot-metrics:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
 
@@ -73,7 +73,7 @@ jobs:
           tox -e mot-py39
 
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
 
@@ -101,7 +101,7 @@ jobs:
 
   install:
     needs: [build]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       max-parallel: 5
       matrix:
@@ -130,7 +130,7 @@ jobs:
   # Executed only if GitHub release is "published".
   #
   release:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     needs: [unit-tests, mot-metrics]
     if: github.event_name == 'release'
     env:

.github/workflows/docs.yml

@@ -16,7 +16,7 @@ concurrency:
 jobs:
   # Single deploy job since we're just deploying
   deploy:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Checkout
         uses: actions/checkout@v3

.github/workflows/lint.yml

@@ -4,7 +4,7 @@ on: [pull_request]
 
 jobs:
   linting:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Check out repository
         uses: actions/checkout@v3

.github/workflows/release-drafter.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   update_release_draft:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: release-drafter/release-drafter@v5
         env: